1. Field of the Invention
This invention relates to a thermoelectric material and to a thermoelectric module using this thermoelectric material. In particular, this invention relates to a thermoelectric material comprising, as a main phase, a half-Heuslar compound having an MgAgAs-type crystalline structure and to a thermoelectric module using this thermoelectric material.
2. Description of the Related Art
In recent years, due to ever increasing consciousness about the global environment, there is increasing interest in a thermoelectric module where the Peltier effect is taken advantage of, such as a flonless refrigerating machine. Likewise, with a view to minimize the discharge of carbon dioxide, there is increasing interest in a thermoelectric module where the Seebeck effect is taken advantage of for providing a power generating system using waste heat energy.
These thermoelectric modules where the Peltier effect or Seebeck effect is taken advantage of are generally created using p-type elements comprising a p-type thermoelectric material and n-type elements comprising an n-type thermoelectric material, these p- and n-type elements being alternately connected with each other in series. Most thermoelectric materials utilized near room temperature are formed of a single crystal or polycrystalline Bi—Te-based material, since the single crystal or polycrystalline Bi—Te-based material is high in conversion efficiency. And thermoelectric materials utilized in above room temperature are usually formed of a single crystal or polycrystalline Pb—Te-based material, since the single crystal or polycrystalline Pb—Te-based material is also high in conversion efficiency.
However, selenium (Se) and lead (Pb) to be employed as a dopant in the Bi—Te-based crystal are noxious and harmful to the human body and also undesirable from a global environmental viewpoint. Therefore, a harmless material which can be employed in place of the Bi—Te-based or Pb—Te-based materials is now being sought.
The half-Heusler compounds can be represented by a chemical formula ABX and is an intermetallic compound having an MgAgAs type cubic crystal structure wherein the B atom is inserted into the NaCl type crystal lattice of AX. Compounds having such a structure are capable of exhibiting a high Seebeck coefficient at room temperature. For example, it is reported that TiNiSn has a Seebeck coefficient of −142 μV/K; ZrNiSn, a Seebeck coefficient of −176 μV/K, and HfNiSn, has a Seebeck coefficient of −124 μV/K.
The figure of merit Z of the thermoelectric material can be represented by the following expression (1):Z=α2σ/κ  (1)
wherein α is the Seebeck coefficient of the thermoelectric material; σ is the conductivity; and κ is the thermal conductivity of the thermoelectric material. The inverse number of the conductivity σ can be represented by the electric resistivity ρ.
The figure of merit Z has a dimension which is the inverse of temperature. When this figure of merit Z is multiplied by the absolute temperature, it gives a dimensionless value ZT. This dimensionless value ZT is called the dimensionless figure-of-merit and is correlated to the thermoelectric conversion efficiency of the thermoelectric material, so that the greater ZT becomes in a thermoelectric material, the greater the thermoelectric conversion efficiency of the thermoelectric material becomes. In other word, provided that a thermoelectric material is less capable of transmitting heat, better at transmitting electricity, and greater in thermoelectric power generating capacity, the thermoelectric material becomes greater in thermoelectric conversion efficiency. For example, the Bi—Te-based material which are the highest dimensionless figure-of-merit among the materials known to date is capable of exhibiting a dimensionless figure-of-merit of about 1.0 at 300K.
Although the aforementioned half-Heuslar compound ZrNiSn has a Seebeck coefficient as high as −176 μV/K at room temperature, its resistivity at room temperature is as high as 11 mΩcm and its thermal conductivity is as high as 8.8 W/mK. Accordingly, it is reported that the dimensionless figure-of-merit ZT of the compound ZrNiSn is as small as 0.010 and the thermoelectric conversion efficiency is also small. In the cases of the compounds TiNiSn and HfNiSn, the thermoelectric conversion efficiency is much smaller, i.e., about 0.007 in the case of TiNiSn and 0.005 in the case of HfNiSn.
There is also reported another half-Heuslar compound which includes all of Ti, Zr and Hf and can be represented by the formula of TixZryHfzNiSn (x=0.5-0.8; y or z=0.1-0.4).
This half-Heuslar compound represented by TixZryHfzNiSn (x=0.5-0.8; y or z=0.1-0.4) is reported to exhibit at room temperature a Seebeck coefficient of −253 μV/K, an resistivity of 6.9 mΩcm and a thermal conductivity of 5.7 W/mK when x, y, and z is 0.5, 0.2 and 0.3, respectively, indicating that since this half-Heuslar compound includes all of Ti, Zr and Hf, its thermal conductivity can be reduced compared with the aforementioned ternary compounds.
However, even with this half-Heuslar compound represented by TixZryHfzNiSn (x=0.5-0.8; y or z=0.1-0.4), the dimensionless figure-of-merit at room temperature is as small as 0.05 or not as high compared with the aforementioned Bi—Te-based material.